A battery is a device in which chemical energy is stored; this energy is then supplied, on demand, as electrical energy, to operate devices during a given period of time, for example to supply electronic devices or electric motors, in particular for means of transport and motor cars.
A battery is generally constituted by one or more electrochemical cells, each of which contains a cathode (positive electrode), an anode (negative electrode) and an electrolyte system.
Rechargeable batteries, also called electrical energy accumulators or secondary batteries, are electrochemical devices in which chemical energy is converted into electrical energy and vice versa, with a charge that can be restored through the application of suitable electrical energy from an external source. These batteries include, for example, Lead-Acid, Nickel-Cadmium, Nickel-Metal Hydride, Alkaline and Lithium Ion batteries.
In particular, lithium ion batteries are characterized by high specific energy, high efficiency, no “memory effect”, and by high usage time. These properties, peculiar to lithium ion batteries, have made them particularly popular for mobile electronic consumer goods, such as cellular telephones, portable computers, digital cameras, MP3 players, etc.
A schematic illustration of the charging and discharging processes that take place in a conventional cell is shown in FIG. 1 (prior art).
The most conventional composition of a lithium-ion battery includes an anode 100 (negative electrode) based on graphite 101, a cathode 200 (positive electrode) based on a lithium metal oxide, generally lithium cobalt oxide, LiCoO2, where anode and cathode are separated by an electrolyte system 300 composed of a solution of a lithium salt, generally lithium hexafluorophosphate, LiPF6, in a mixture of aprotic organic solvents, prevalently ethylene carbonate and dimethyl carbonate.
Conventional lithium ion batteries are charged and discharged through the reversible transport of lithium ions Li+, in the organic electrolyte medium, between the anode and the cathode, with consequent electron exchange. In particular, the ions move from the anode to the cathode during discharging and from the cathode to the anode during charging. It must be noted that only the lithium ions Li+ participate in the charge/discharge reactions.
To ensure development of the lithium-ion battery technology, also in view of application in emerging markets, such as accumulation in power plants based on discontinuous sources (for example solar and/or wind energy) and sustainable road transport with electric and/or hybrid cars, it is necessary to renew the electrochemical system of these batteries in order to increase their energy content and safety level and to decrease their cost.
Among the alternative cathode materials to conventional LiCoO2, compounds of the type LiM1M2O4 are known, where M1 and M2 are metals selected from Mn, Ni, Fe, Co, P or combinations thereof. Classic examples are nickel manganese spinel LiNi0.5Mn1.5O4 and lithium iron phosphate LiFePO4. With regard to the alternative anode materials to graphite, compounds formed of lithium alloys, such as LixSn, LixSi, LixSb, are known.
Moreover, electrolyte systems alternative to liquid organic systems are also known, such as room temperature molten salts (ionic liquids) like those based on pyrrolidinium or solid or gel polymers such as those based on polyethylene oxide (PEO) or polyvinylidene fluoride (PVdF).
However, also in these cases, the electrolyte systems embed a single metal ion, which is the only one to be involved in the electrochemical process.
The prior art concerning materials for lithium ion accumulators refers prevalently to their use in conventional systems, both for the use of the electrolyte (organic liquid solutions or polymers) and of the anode material (graphite or others). However, these systems are disadvantageously characterized by the presence of inflammable organic materials (electrolytes) that cause operating risks (safety of the battery) and high costs associated both with the processing of prototypes and with their operation. The preparation of prototypes must in fact be performed in extremely controlled environments in terms of humidity and oxygen content, which must be maintained at levels of a few parts per million.
Their operation is influenced by an extreme sensitivity to atmospheric agents, which can cause fires and, in extreme conditions, also explosions. Further factors that influence the costs are associated with the production of solvents and organic electrolytes commonly used in commercial lithium ion batteries, and with the need to use complex electronic systems (for example BMS, battery management systems) to control charging of the single cells and simultaneous equalization of several cells.
The main disadvantage of current lithium ion batteries therefore resides in the electrolyte system used.
Moreover, current technology does not allow the production of lithium ion batteries of large dimensions which are also safe, suitable to store large quantities of energy, capable of compensating for the difference in demand for energy of electricity networks between day and night. This is due to the high reactivity of the common organic electrolytes with the materials of which the electrodes are made, which causes serious safety problems, in particular in reproducing lithium ion batteries with large dimensions.
The document TAKEGAMI ET AL: “Aqueous Lithium Ion-Zinc Secondary Battery”, KÔEN-YÔSHISHÛ, DENKI-KAGAKKAI-74.-TAIKAI: KÔFU, Mar. 29-31, 2008, ELECTROCHEMICAL SOCIETY OF JAPAN, JP, no. 10, 1 Jan. 2006 (2006 Jan. 1), pages 825-827, XP008163301, ISSN: 1344-3542 discloses a rechargeable electrochemical cell composed of two coupled independent half-cells (anode and cathode system), wherein the side of the negative electrode (Zn) and the side of the positive electrode (LiMn2O4) are physically separated. The two half-cells are in fact characterized respectively by different acidic and basic conditions, i.e. by different pH, which makes their mutual separation essential.
In particular, separation is obtained by means of a separator made of nonwoven fabric in polyester fiber and plastic material.